Studying the latent reservoir in HIV-1 infected individuals is critical because it is a major barrier to cure. The most clearly defined latently infected cells in vivo are resting CD4+ T cells. Latently infected resting CD4+ T cells are assumed to be the source of viral reactivation when antiretroviral therapy is interrupted. In HIV-1 infected individuals who are maximally suppressed on antiretroviral therapy, approximately 1 in 10(4) resting CD4+ T cells contain integrated proviral DNA, yet only about 1 in 10(6) cells produce virus upon stimulation- i.e. contain latent provirus. What explains the large discrepancy between the amount of integrated HIV-1 proviral DNA and the percentage of cells that are latently infected? What determines whether proviral integration leads to defective versus latent infection? These questions are difficult to answer using cells from HIV-1 infected individuals, because the frequency of latently infected cells is low. The mechanisms that drive a provirus toward defective versus latent infection are undefined. Understanding how proviruses are rendered defective may lead to therapies that decrease the size of HIV-1 reservoirs. Our ability to achieve a high frequency of latent infection and to separate defective from latently infected cells allows us to answer questions that cannot be studied using in vivo samples. We have developed an in vitro model of HIV-1 latency in resting CD4+ T cells that overcomes the low frequency of latent infection that is present in vivo and allows us to study mechanisms of proviral inactivation versus proviral latency. We propose to study three aspects of proviral integration in resting CD4+ T cells that may influence the defective and latent state. They are: (1) the frequency of multiply infected cells which gives the appearance of inactivated proviruses (2) the frequency of mutations in defective and latent proviruses (3) the integration site selection of defective and latent proviruses.